Page 380 - Pipeline Pigging Technology

High-accuracy calliper surveys

Feature summary

Because of the accuracy of the calliper sonar, a large number of potential
features show up. Most of the objects that show up in the smaller sizes are
normal, such as the slight out-of-roundness in bends, ovality in deep overbur-
den, and changes in wall thickness. In the largest sizes, all the sensor drop-outs
and bad readings show up. Although the number of these is significant
compared to the dents, when considering the fact that the 72 sensors are
firing 64 times a second for several days, it can be seen that there are not many
spurious readings.

Statistics

A typical section 200km (125 miles) in length contains over 1500 places
where the total out-of-roundness or deviation from the ideal circular shape
exceeds 1 cm (3/8in). As one would expect, the overall average clock position
of these is at 6 o'clock, or on the bottom. The average length is 0.5m and the
average height is 1.5cm. The distribution shows over three quarters lie
between 1 and 2cm (less than 3% of nominal diameter).

Data spikes

The single data spikes that were observed in the early runs were, to a large
extent, removed by changing the timing and power levels. Later runs have
shown very few spikes, less than 40 in 72 hours run time.

Dent verification

Five sites were dug up to verify the location and accuracy of the Geoptg
measurements. Although there was a month time lag between the measure-
ments and the overburden had been replaced when the internal measure-
ments were taken, all five dents compared within 1.5mm (or 60 thousandths
of an inch). The ovality had increased in two of the cases from the dig to the
internal measurement as might be expected. Table 2 compares the results of
the test dig program with the Geopig measurements.

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